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©1995-99 Exploratorium / Science Learning Network

The Cow's Eye Dissection is one of the most popular demonstrations at the Exploratorium.
For many years it has helped people satisfy their curiosity about what is inside an eye.
The material presented here is meant not to replace the act of dissecting a cow's eye,
but rather to enhance the experience.

Before the dissection: Try this!

Shape your hand like an eye socket, place the eye in it, and pull on the muscles that are attached to the eye. It helps to demonstrate that cows have four muscles that control their eyes, while we have six, which allows us to "roll" our eyes.

Step 1

Examine the outside of the eye. See how many parts of the eye you can identify. You should be able to find the whites (or sclera) and the clear covering over the front of the eye (the cornea). You should also be able to identify the fat and muscle surrounding the eye.

Step 2
Make the first incision where the sclera meets the cornea. Cut until the aqueous humor is released.

Step 3
Rotate the eye and cut around the cornea. Be careful not to cut too deep or you may cut the lens. As the cornea starts to cut free, hold the cornea in the center and make the last cuts around it.

Step 4:
Once you have removed the cornea, place it on the board (or cutting surface) and cut it with your scalpel or razor.
Try this!
Examine the convex nature of the cornea. Can you guess how the cornea holds this shape?
Hint: If you are having trouble, find out more about the aqueous humor.

Step 5:
With the cornea removed, the next step is to pull out the iris. Place one finger in the center of the eye. Find the iris and pull it back. It should come out in one piece.

Step 6:
It can be a bit tricky to remove the lens with the vitreous humor attached. It works best if you cut slits in the sclera. Be careful not to cut the lens.

Step 7
After enough incisions have been made in the sclera, you should be able to remove the lens. Sometimes the vitreous humor will be removed along with the lens.

Hold up the lens and look through it. If the lens is too slippery, pat it dry and try again.

Try this!
Look at a piece of paper through the lens and try to read the words. You should notice that the images appear upside down. All the information that comes into the eye gets turned upside down and backward by the lens.

Step 9
With the vitreous humor now removed, you should be able to turn the eye inside out.

Step 10
The thin tissue on the back of the eye is the retina. Find the spot where the retina is attached. The shiny blue-green material is the tapetum.

Step 11
Find the spot where all the retina's nerves collect. It is called the blind spot. This is where all the nerves go out the back of the eye, forming the optic nerve.

Step 12
Return your attention to the outside of the eye. Locate the optic nerve. To see the separate fibers that make up the optic nerve, pinch the nerve with a pair of scissors or with your fingers.

Once the dissection is complete, properly dispose of the remains. They should be wrapped up in plastic and disposed of the same day. Also, if you used razor blades, they should be disposed of properly. (A blade is good for only one or two dissections.)

Interesting Facts!

* The fat that surrounds the eye is there for a reason. It helps cushion the eye and protect it from the hard bone of the eye socket.

* A cow's cornea has about seven or eight layers of material. We have three to five layers. The cow needs these extra layers of protection because it spends so much time grazing close to the ground, where its eyes could be damaged by sticks or other objects. The lens does only about 20 percent of our focusing. The cornea does the other 80 percent. The lens changes shape so that you can focus on things that are near and things that are far away. The ciliary body controls the shape of the lens.

* The iris is the part of the eye that gives us brown, blue, or green eyes. All cows have brown eyes.
The lens grows layers like an onion. As you get older, the lens becomes less flexible because the buildup of layers compacts the center of the lens, making it more rigid. When the lens becomes less flexible, it can't change shape to focus on things nearby. This is why many people need glasses as they get older.

* The shiny blue-green tapetum helps the cow see at night. Many other animals have a tapetum. You may have seen their eyes glow when your car's headlights flash on them. The tapetum helps animals see at night by reflecting the light entering the eye back at the retina a second time.

One difference between the cow's eye and the human eye is the shape of the pupil. The cow's pupil is oval. Our eyes have round pupils.

* Another difference between the cow's eye and the human eye is that cow's can't see color only shapes. Cow's can't see colors because they lack cones in their retina.

of Terms!

Aqueous humor

A clear fluid that helps the cornea keep its rounded shape.
Blind spot
The area where the optic nerve leaves the retina. Each eye has a blind spot where there are no photoreceptor cells.
Blood vessels
Tiny arteries and veins that carry blood to the retina.
Ciliary body
Muscles that control the shape of the lens for near and far vision.
One type of photoreceptor cells in the retina. They are responsible for daylight and color vision.
A clear, tough covering over the iris and the pupil that helps protect the eye and begins focusing the light.
A dimple in the retina where cones are concentrated and vision is most acute.
A muscle that controls the amount of light that enters the eye. It is suspended between the cornea and the lens.
A clear, flexible structure that adjusts the eye's focus, allowing us to see objects both near and far. It is responsible for about 20 percent of our focusing.
Optic nerve
The bundle of nerve fibers that carry information from the retina to the brain.
The layer of light-sensitive cells lining the inner eyeball. It detects images focused on the back of the eye by the lens and the cornea. The retina is connected to the brain by the optic nerve.
One type of photoreceptor cells in the retina. They respond to dim light.

The thick, tough, white outer covering of the eyeball.
Suspensory ligaments
Fibers that connect the ciliary body to the lens.
The colorful, shiny material located behind the retina. Found in animals that have good night vision, it reflects light back through the retina.
Vitreous humor
The thick, clear jelly that helps give the eyeball its shape.